Although homeless youth (HY) are particularly vulnerable to alcohol and other drug abuse, they constitute one of this Nation's most undeserved populations. In this proposed research, we are requesting support for a two-year study with four major aims, as follows: to estimate the relative prevalence of drug use among HY in shelters, homeless street youth, and nonhomeless youth in households; to describe the links between drug use by HY and their gender, age, race, length of time away from home; whether they have dropped out of school; and whether they have prostitution experiences; to model the collection contributions to HY's drug use of their family composition and socioeconomic status, depression (including suicide attempts), household members' alcohol and other drug use, conflict with household members and problems with friends at school; and to describe HY whose drug use co-occurs with multiple criminal behaviors including theft, assault, and drug dealing. This study will consist of a secondary analysis of data obtained from two studies, on of HY, entitled "Incidence and Prevalence of Drug Abuse among Runaway and Homeless Youth," and the other of youth in general, entitled the "National Household Study of Drug Abuse." The former comprises interviews with 640 youth residing in a nationally representative sample of youth shelters and a purposive sample of 600 street youth. The latter includes interviews with 12,000 household youth. Comparisons will be made using logistic regression modelling of drug used across all three populations, adjusting for differences in youth's age, gender, and race in the three samples. A similar strategy will be used to test all three aims. Study results will provide an invaluable asset to the planning of drug treatment services that meet the needs of this elusive and understudied population and key vulnerable subpopulations within it, and to demonstrating the nature and extent of the need for such services.